The present invention pertains generally to optical fiber communication systems, and more particularly, to a high-precision etalon for use in monitoring the frequency of an optical laser and method of construction thereof.
Optical communication systems utilize optical fibers as information carrying channels. Their low attenuation properties make optical fibers a high-quality transmission medium and allow them able to achieve high data rates. Information is transmitted onto and received from an optical network via optical links which implement a transmitter, receiver, or both. An optical transmitter is implemented with a laser, which is a very high frequency optical oscillator constructed from an amplifier and an appropriate amount of positive feedback.
An etalon is the passive resonant structure often used as an optical filter to establish the wavelength of a transmitted laser beam at a predefined frequency. As known by those skilled in the art, an etalon is an optical resonator comprising two plane-parallel, flat mirrors placed a fixed distance apart. Because its intercavity spacing is fixed, the peak transmission frequencies defined by a particular etalon are unchangeable. The etalon is characterized by a series of equally spaced resonant frequencies that cause it to operate as a comb filter when used as a transmitter. Hence, the etalon is an ideal device for ensuring the proper separation of channel frequencies on a multiple-channel high-resolution optical fiber.
FIG. 1 is a schematic diagram of a conventional etalon optical filter (hereinafter xe2x80x9cetalonxe2x80x9d) 2. Etalon 2 includes two plane-parallel partially transmissive feedback mirrors 4a and 4b positioned to form a cavity 6 therebetween. Mirrors 4a and 4b are formed as a pair of flat transparent substrates 12a and 12b to each of which a reflective coating 14a and 14b has been applied. It is common practice to form the substrates 12a, 12b as slightly wedge-shaped and to coat the outer faces of substrates 12a and 12b with an anti-reflective layer to prevent the substrates themselves from acting as optical resonators
Cavity 6 is typically air-filled, or may comprise a crystalline or glassy solid, insulating material, or semiconductor material When used as an optical filter, a laser beam is transmitted through one of the mirrors 4a, 4b, which reflects between the mirrors 4a and 4b in the etalon cavity 6, resulting in constructive interference and a repetitive series of high transmission spikes. The peak transmission of light passing through the etalon 2 occurs at resonant frequencies fm.
The transmission characteristics of the etalon 2 are illustrated in FIG. 2. The frequencies fm of maximum transmission satisfy the equation:             f      m        =                  m        ⁢                  xe2x80x83                ⁢                  c          0                            2        ⁢        nl        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢        θ              ,
where m is an integer, c0 is the velocity of light in a vacuum, n is the refractive index of the cavity 6, l is the spacing between the mirrors 4a and 4b, and xcex8 is the angle of refraction of the incident light, and xcex is the wavelength of the light between the reflectors 4a and 4b. Adjacent frequencies at which the etalon shows maximum transmission are separated by a frequency.             Δ      ⁢              xe2x80x83            ⁢      f        =                  c        0                    2        ⁢        n        ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢        θ              ,
where xcex94f is called the free spectral range (FSR) of the etalon. The frequencies fm of maximum transmission are equally spaced. A device that has this characteristic is called a comb filter.
The free spectral range FSR is constant for a given wavelength, which is dependent upon the spacing l of the gap of cavity 6. Techniques such as wedge tilting exist, in which the angle of the incident light is adjusted, for shifting the maximum transmission frequencies fm a small degree (e.g., less than 15xc2x0). Accordingly, in applications which require a precise FSR, the spacing l of the gap of the cavity 6 of the etalon 2 must be within very tight tolerance limits.
FIG. 3 illustrates a typical application of an optical filter etalon. In particular, FIG. 3 is a block diagram of a portion of a fiber optic network that employs an etalon 2 to monitor the wavelengths of a multiple-channel optical fiber 18. In this application, the goal is to lock the frequency of the transmitted laser beam to a frequency that overlaps one of the resonant frequency fm of the etalon 2.
In operation, a laser 15 generates a laser beam 25 at a frequency fx. Laser beam 15 is transmitted onto optical fiber 18a, which passes through a coupler 5. Coupler 5 couples most of the signal (e.g., 90%) onto optical fiber 18b, which couples to optical cable 18 for transmission to other destinations Coupler 5 couples the remaining portion of the laser beam signal 25 onto optic fiber 18c, which is received by wave blocker 16. Wave blocker 16 comprises a lens 17, etalon 2, and a detector 19. Lens 17 focuses the laser beam 25 received from optic fiber 18c for transmission through etalon 2. Etalon 2 filters the received beam 25 according to its resonant frequencies fm. Detector 19 detects whether or not the frequency fx of the laser beam 25 overlaps one of the etalon""s resonant frequencies fm. The detector output is used by a laser frequency controller 9 to adjust the output frequency fx of the laser beam 25.
The etalon filters are manufactured such that the resonant frequencies fm overlap the standard channels set up and regulated by the Federal Communication Commission (FCC) or International Telecommunications Unions (ITU) for fiber optic transmission. The standard channel definition provides for the communication of data across optical fibers between local, national, and even international data exchanges.
Etalons must be manufactured with a cavity gap 6 spacing l of a precise length to allow the maximum transmission frequencies fm (i.e., the transmission peaks in FIG. 2) to overlap the defined standardized channel. Although the exact values of the maximum transmission frequencies fm can be changed slightly via etalon wedge tilting techniques, if the spacing xcex94f of the transmission peaks is not exact, it is impossible to line up all the peaks to overlap with the defined channels even with tilting.
Recently, a strong commercial interest has centered around the use of temperature stable etalons for telecommunications applications in the 1.55 um wavelength range. To achieve the high level of required temperature stability, these etalons are typically manufactured using mirrors separated with a fixed air gap whose length is determined by a precision polished spacer made from a low thermal expansion material such as ULE or Zerodur. For the intended applications, the etalons must also have very tight control over the effective spacing l of the mirrors. This requires that the polishing process in which the spacers are fabricated be controlled precisely in order to hold the finished length of the spacers to within typically a few fractions of a micron or less of the desired value l. In a volume manufacturing process this requirement poses serious difficulties in terms of both metrology and process control, causing the manufacturing process to be very expensive. Specifically, because of the difficulty in controlling the polishing process to the degree of accuracy required in these applications (i.e., to within a fraction of a micron), the current wafer polishing technique cannot meet the demands for manufacture of high-precision etalons. Accordingly, a need exists for an alternative high-volume method for manufacturing high-precision etalons.
The present invention is a novel high-precision manufacturing process that greatly facilitates high-volume economic manufacture of air spaced etalons in which the mirror spacing must be controlled within a fraction of a micron. The invention utilizes coating deposition processes, which are fundamentally easier to monitor and control than polishing processes, to allow fine control over cavity gap spacing.
In accordance with one embodiment of the invention, a set of spacers or spacer material is fabricated on a wafer using wafer polishing techniques to meet all wedge and flatness requirements for the product (verified interferometrically); however, the thickness of the wafer is left to exceed the nominal dimension of the specified cavity gap space by a small amount that can be confidently measured by precision mechanical tools. The wafer is then removed from the polishing block and diced into spacers.
The length of the spacers is then measured by optical means to a precision better than the tolerance on the final etalon product.
A value is then derived for the dimensional deviation of the spacer length from the specified nominal dimension of the final etalon product.
A thin film deposition process is then used to deposit a transparent thin-film pedestal on at least one of the substrates that are used to fabricate the mirrors of the etalon. The physical thickness of this layer is equal to half of the dimensional difference between the actual spacer length and the nominal specified gap dimension. Typically the deposition process can provide control over the layer thickness to a level of 0.01 um or better, which exceeds the tolerance of the cavity gap length required of current commercial products. This layer covers only that portion of the mirror substrate that is open to the air gap; it does not cover that portion of the mirror substrate that comes into contact with the spacer. Accordingly, a thin-film pedestal is built up on the substrate. A reflective coating is then deposited on top of the thin film layer.
The etalon is then assembled from its constituent pieces. Due to the deposition of the adjustment thin film layer, the spacing of the mirrors is precisely equal to the desired nominal value, even though the thickness of the spacer may deviate from the nominal by an amount substantially larger than the tolerance on the mirror spacing.
An analogous process in which the spacer is made slightly smaller than the nominal dimension provides for the deposition of the thin film thickness adjustment layer directly on the spacer.